It is well known that, in any space having a recycled air system, it is highly desirable to filter the recycled air to remove particulate matter. However, some air contaminants are gaseous in nature and are not easily removable by filters that rely on interception or entrapment of particulates carried by the air. Some of these gases may be toxic, noxious, unpleasant to smell, or they may be merely undesirable in a specific situation such as water vapor in a humid environment or an elevated carbon dioxide concentration in recirculated air, or otherwise undesirable (such gases being referred to generally as "undesirable" in what follows) and a means for their removal in such a system is highly desirable. The problem is particularly acute in airplanes, or other closed environments such as submarines, fall-out shelters, and space craft, where the recycling of air containing undesirable odors is not calculated to produce contented occupants. It is also a significant problem if minor amounts of toxic or irritant gases are present, or being liberated, as in a chemical warfare situation or an environment accidentally contaminated with hazardous gases. Recycling air that is merely cleaned of particulate contamination will increase the exposure to such gases and, therefore, increase the risk of adverse consequences.
It is known that some materials are highly effective at sorbing gases and it has been proposed that filtered air also be passed through a pad of one of these materials before completing the recycle operation. This is a satisfactory expedient up to a point, that is, there are no problems until the sorbent material is saturated and gas "breakthrough" occurs. From a clean flow, the change to a flow containing undesirable gases is swift and dramatic because the air flow is, by design, equal through all parts of the pad so far as can be arranged. Thus, breakthrough occurs at all points essentially simultaneously. Since it is not easy to predict or measure the approach of breakthrough due to the unknown challenge encountered by the filter, it is necessary to have excessively frequent pad changes or risk breakthrough occurring at an inconvenient time such as, (in a cabin-air situation), the beginning of a long non-stop transpacific flight.
Another problem that is associated with sorption filter pads is that they frequently cause an undesirably high pressure drop across the filter pad such that high power rating air circulation equipment may be needed with the associated problems, not only of power consumption, but also of size and noise.